Observation and photoelectron spectroscopic study of novel mono- and diiron oxide molecules: FeOy- (y=1-4) and Fe2Oy- (y=1-5)

We study novel iron oxide molecules involving one and two iron atoms by anion photoelectron spectroscopy at 3.49 and 4.66 eV photon energies. Vibrationally resolved photoelectron spectra and low-lying excited states are obtained for FeOy- (y = 1-4) and Fe2Oy- (y = 1-5). In both series, the photoelectron spectra became particularly sharp and better resolved for the higher oxides, FeO3-, FeO4-, Fe2O4-, and Fe2O5-. The electron affinity of the neutral oxide molecules is observed to increase with the number of oxygen atoms, suggesting a sequential oxidation behavior. For the monoiron oxide series, an oxidation saturation is observed as evidenced by the leveling-off of the electron affinity from FeO3 to FeO4. The structures and chemical bonding of these oxide molecules are discussed based on the sequential oxidation behavior. Isomers involving possibly O-2 or O-3 complexes are also observed for the diiron oxides above Fe2O.